| Orchards Woodside Management Company Limited | Households | 2017 |
| Orchid Pavilion Limited | Hospitality | 2018 |
| Orchis Solutions Limited | Information & communication | 2000 |
| Ordologica Ltd | Information & communication | 2026 |
| Oree Technologies Ltd | Information & communication | 2025 |
| Orensanz Consulting Limited | Professional & scientific | 2024 |
| Organford Manor Country Park Limited | Hospitality | 2014 |
| Organic Stuff Limited | Households | 2017 |
| Ori.Ai Ltd | Professional & scientific | 2024 |
| Orian (Middle East) Limited | Professional & scientific | 2013 |
| Oriens Custos Ltd | Manufacturing | 2025 |
| Oriental Civil Engineering & Construction Co., Ltd | Professional & scientific | 2017 |
| Oriental Yifan Co., Ltd | Manufacturing | 2023 |
| Origami Applied Technology Ltd | Information & communication | 2024 |
| Original AI Ltd | Real estate | 2024 |
| Original Binary Refinery Ltd | Information & communication | 2022 |
| Original Futures Ltd | Professional & scientific | 2008 |
| Originita Ltd | Wholesale & retail | 2019 |
| Origins Films Ltd | Information & communication | 2015 |
| Orinoco Resources Ltd | Professional & scientific | 2016 |
| Orion Crest Ltd | Real estate | 2026 |
| Orion Energy Solutions Ltd | Construction | 2011 |
| Orion Innovation Solutions UK Limited | Information & communication | 2000 |
| Orion Insulation Ltd | Extraterritorial | 2015 |
| Orion Medtech Ltd C.I.C. | Information & communication | 2018 |
| Orion Tax LLP | | 2025 |
| Orka Landscapes Ltd | Administrative & support | 2026 |
| Orlenes Kitchen Cafe Limited | Hospitality | 2025 |
| Orlovnjak Ltd | Professional & scientific | 2020 |
| Orome Ltd | Professional & scientific | 2025 |
| ORR Advisory Limited | Professional & scientific | 2025 |
| Orris Construction Ltd | Construction | 2022 |
| Orriss Aviation Ltd | Manufacturing | 2023 |
| ORS Trade Ltd | Wholesale & retail | 2025 |
| Orsfx Limited | Financial services | 2025 |
| Orsho Ltd | Professional & scientific | 2021 |
| Orthobio Therapeutics UK Limited | Health & social care | 2022 |
| Orthogonal Research Limited | Information & communication | 2025 |
| Orton Civil Design Ltd | Professional & scientific | 2015 |
| Oryam Ltd | Extraterritorial | 2026 |
| Orycteropus Consulting Ltd | Professional & scientific | 2018 |
| Oryginate Limited | Wholesale & retail | 2011 |
| OS Transformation Limited | Professional & scientific | 2025 |
| Osborn Mcnamara Ltd | Information & communication | 2025 |
| Oscar's Lockdown Pizza Limited | Hospitality | 2022 |
| Osimhen Ltd | Wholesale & retail | 2026 |
| Osipo Properties Limited | Real estate | 2024 |
| Osiris-Ai Limited | Information & communication | 2020 |
| Osisoft (UK) Limited | Information & communication | 2010 |
| Oska Marketing Ltd | Professional & scientific | 2021 |
| Osmanthus Trading Company Ltd | Wholesale & retail | 2023 |
| Osny Link Co., Ltd | Agriculture & fishing | 2025 |
| Ososim Limited | Education | 2008 |
| OSP Building Ltd | Construction | 2025 |
| Osprey Consulting Services Limited | Professional & scientific | 2006 |
| Osprey Court (Huntingdon) Management Limited | Households | 2003 |
| Osprey Drive Cambridge Ltd | Households | 2018 |
| Osprey Lettings Ltd | Real estate | 2023 |
| OST Marketing Limited | Professional & scientific | 2010 |
| Ostouri Ltd | Real estate | 2021 |
| Othello Sealing Material UK Ltd | Manufacturing | 2025 |
| Othersyde Ltd | Hospitality | 2017 |
| Otium Consulting Limited | Real estate | 2018 |
| Otos Digital Ltd | Information & communication | 2026 |
| Otrading Ltd | Information & communication | 2025 |
| OTS Coding Ltd | Professional & scientific | 2025 |
| Ottaggio Property Advisory Ltd | Real estate | 2026 |
| Otters Reach (Surfleet) Management Company Ltd | Households | 2020 |
| Otterwind Trading Ltd | Wholesale & retail | 2025 |
| Otzi Group Ltd | Information & communication | 2025 |
| Oudafei Trading Co., Limited | Extraterritorial | 2024 |
| Ouens Ltd | Education | 2024 |
| Ounce Technology Limited | Information & communication | 2010 |
| Ounje Adun Ltd | Hospitality | 2023 |
| Ounuo Group Limited | Extraterritorial | 2017 |
| Oupai Industrial Co., Ltd. | Extraterritorial | 2012 |
| OUR Legacy Sustainability Consulting Limited | Professional & scientific | 2021 |
| OUR Unibridge Limited | Education | 2024 |
| OUT of House Limited | Information & communication | 2006 |
| Outdoor Winner Industries Co., Ltd | Extraterritorial | 2014 |
| Outlets Digital Economy Global Alliance | Wholesale & retail | 2025 |
| Outline Building and Property Ltd | Construction | 2013 |
| Output Design Limited | Information & communication | 2015 |
| Outsee Limited | Professional & scientific | 2022 |
| Outside in (Cambridge) Limited | Manufacturing | 1991 |
| Outsourced Business Limited | Professional & scientific | 2017 |
| Outsourco Ltd | Information & communication | 2025 |
| Outspoken Cycles Ltd | Wholesale & retail | 2012 |
| Outspoken Delivery LLP | | 2005 |
| Outspoken Property Ltd | Real estate | 2015 |
| Outspoken Training LLP | | 2010 |
| Oval Medical Limited | Extraterritorial | 2008 |
| Oval Medical Technologies Limited | Professional & scientific | 2009 |
| Oval Resources Advisors Limited | Professional & scientific | 2016 |
| Ovation Bidco Ltd | Financial services | 2019 |
| Ovation Midco Ltd | Financial services | 2019 |
| Over & in Football Limited | Education | 2019 |
| Over & in Limited | Education | 2012 |
| Over & in Tennis Limited | Arts & recreation | 2022 |
| Over Border Branding Studio Ltd | Professional & scientific | 2025 |